A paper by U. Balachandran et al (Argonne National Laboratory), "Synthesis of Phase-Pure Orthorhombic YBa.sub.2 Cu.sub.3 O.sub.x Under Low Oxygen Pressure" in Materials Lett., 8, 454 (1989) discloses milling a mixture of Y.sub.2 O.sub.3, BaCO.sub.3, and CuO in methanol; drying and screening the resultant slurry; and calcining the screened powders for 4 hours in flowing O.sub.2 with a pressure of 2-20 mm Hg and at a temperature of 800.degree. C. The product was cooled under flowing O.sub.2 at ambient conditions, with a 3-hour hold at 450.degree. C. to promote oxygenation of the resulting powder. Under optimum conditions (2.7.times.10.sup.2 Pa which is 2 mm Hg) the powders were phase pure. Particle size was 1-4 microns. Pellets sintered in O.sub.2 at 915.degree.-980.degree. C. gave densities of 90-96% of theoretical.
In a related paper U. Balachandran et al describe the preparation of YBa.sub.2 Cu.sub.4 O.sub.8 by a similar process: "Synthesis of 80 K Superconducting YBa.sub.2 Cu.sub.4 O.sub.8 Via a Novel Route" in Physica C., 165, 335 (1990).
Similar powders made by the process of the instant invention provide densities at least as high as those of Balachandran, but made at considerably lower temperatures (typically 700.degree.-750.degree. C.). The ability to achieve YBa.sub.2 Cu.sub.3 O.sub.X at lower temperatures is believed to result both from the much smaller volume median particle size (i.e., submicron versus 1-4 microns for the Balachandran product), and the much greater homogeneity of products of the invention process.